(1) Field of the Invention
This invention relates to an apparatus for use in a laser printer adapted to control a semiconductor laser so as to make this laser emit a constant optical output.
(2) Description of the Prior Art
The intensity of emission from a laser diode relative to a current supplied thereto is highly temperature dependent. For this reason, in a laser printer, APC (automatic power control) is performed each time printing of, for example, one page is completed.
A circuit adapted for this control detects the intensity of emission from the laser diode by means of an optical detector, compares a detection value thereby obtained with a reference value by mean of a comparator, and increases or decreases the current supplied to the semiconductor laser in response to the result of this comparison. This operation is continued for a certain period of time (determined by considering that the control can provide desired effects within this period of time), and the control is thereafter terminated.
However, there is a possibility of occurrence of an error in the result of comparison because of influence on the comparing operation due to noise caused when charge is effected on a sensitive drum in a corona discharge manner. There is therefore a possibility of the emssion intensity control being terminated if the intensity of emission is deviated from a reference value, resulting in deterioration or thermorunaway of the semiconductor laser.
The emission intensity control cannot not be terminated in a short time, and the operation of the printer cannot be changed over speedily to the next printing process.
In addition, it is not possible to confirm whether or not the emission intensity control is completed without any abnormality.